WASHINGTON — Methicillin-resistant strains of Staphylococcus aureus in both healthy and hospitalized children are increasing throughout the country, accompanied by small, but significant, increases in strains that are resistant to both methicillin and clindamycin, according to researchers at the annual meeting of the Pediatric Academic Societies.
Houston has seen ever-increasing rates of methicillin-resistant S. aureus (MRSA) since February 2000, when one-third of community-acquired S. aureus infections were already positive for MRSA, Sheldon Kaplan, M.D., of Texas Children's Hospital, Houston, told this newspaper.
During his poster presentation, Dr. Kaplan said that by November 2000, that rate had increased to 50% and has continued to increase every year. Recently, MRSA isolates that also are resistant to clindamycin have been identified.
His study tracked community-acquired S. aureus infections among pediatric inpatients and outpatients from August 2001 to July 2004. During the study period, 3,578 isolates were associated with community-acquired infections; 74% were MRSA.
In year 1, 72% of the community-acquired S. aureus isolates were MRSA (551 of 771); in year 2, 74% were MRSA (915 of 1,245); and in year 3, 77% were MRSA (1,193 of 1,562). Community-acquired MRSA (CA-MRSA) isolates increased by 2.2-fold, while the increase in community-acquired methicillin-susceptible S. aureus (CA-MSSA) isolates was 1.7-fold.
Of the CA-MRSA isolates, 2,542 (96%) were recovered from children with skin and soft-tissue infection; 62% of these children were admitted to the hospital. Most of the CA-MSSA isolates (92%) were also recovered from skin and soft-tissue infections, with 53% of these children admitted to the hospital.
Most of the CA-MRSA isolates (95%) and about half of CA-MSSA isolates were also resistant to erythromycin; the levels were steady throughout the study. Clindamycin resistance increased among both CA-MRSA and CA-MSSA, although the rates remained low: 2%–6% for MRSA; 3%–11% for MSSA.
This association appears to contradict the theory that the 7-valent pneumococcal conjugate vaccine is related to the increase in community-acquired S. aureus infections, Dr. Kaplan noted.
Although the Texas increase in CA-MRSA occurred in the same year as the PCV7 was licensed for use in children younger than 24 months, there was no significant increase in CA-MRSA infections in that age group during the 3 years of the study. “Our data would not support the idea that the use of PCV7 is associated with the increasing numbers of S. aureus infections we encountered,” Dr. Kaplan said at the meeting, which was sponsored by the American Pediatric Society, the Society for Pediatric Research, the Ambulatory Pediatric Association, and the American Academy of Pediatrics.
CA-MRSA was largely responsible for a dramatic increase in deep venous thrombosis among children hospitalized at the center, according to a poster presented by Blanca Gonzalez, M.D., also of Texas Children's Hospital.
” From 1999 to 2004, we identified 10 children who presented with or developed a venous thrombophlebitis during their hospitalization with an invasive S. aureus infection; nine of those occurred after 2001.”
Of the 10 cases, she said, 8 were associated with CA-MRSA infections and 2 with CA-MSSA infections. Eight of the isolates were positive for Panton-Valentine leukocidin (PVL), a cytotoxin that causes leukocyte destruction and tissue necrosis, and which is produced by most CA-MRSA strains.
“The current PVL-positive clone circulating in Houston appears to have an enhanced propensity to cause DVT in association with osteomyelitis,” Dr. Gonzalez said.
The children ranged in age from 9 months to 14 years. Although six had central catheterization lines, the youngest child was the only one whose DVT was clearly associated with a catheter. The most common site of thrombosis was the femoral vein (60%), and clots frequently extended into the popliteal veins. All of the patients had osteomyelitis and pyomyositis; in nine children, these infections were located adjacent to the site of the thrombosis.
Four patients had septic pulmonary embolisms and required inferior vena cava (IVC) filters. All the children were treated with anticoagulation therapy with either low-molecular-weight heparin or warfarin. Therapy was maintained for a mean of 3.7 months (2.5–7 months).
The thromboses completely resolved in eight patients by a mean of 10 weeks. One child was lost to follow-up and the last child remains on anticoagulation therapy, she said.
In Nashville, Tenn., nasal carriage rates of MRSA are increasing dramatically, said Clarence Buddy Creech, M.D. His study also identified clindamycin-resistant MRSA.
In 2004, Dr. Creech of Vanderbilt University in Nashville studied nasal carriage rates of S. aureus in 500 healthy children, and then compared the rates with a similar study of 500 other healthy children performed at the center in 2001.